This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, the invention relates to an impedance matching device which utilizes mutual inductance to couple an NMR radio frequency (RF) coil to an RF power amplifier and RF receiver preamplifier which form part of an NMR scanner.
The nuclear magnetic resonance phenomonen has been utilized in the past in high resolution NMR spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, NMR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, non-invasive, spectroscopic analyses. As is now well known, the NMR resonance phenomonen can be excited within a sample object, such as a human patient, positioned in a polarizing magnetic field, by irradiating the object with RF energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil and an RF preamplifier are used to receive the NMR signal emanating from the patient volume lying within the field of the RF coil. The NMR signal is usually observed in the presence of linear magnetic field gradients used to encode spatial information into the signal. In the course of a complete NMR scan, a plurality of NMR signals are typically observed. The signals are then used to derive NMR imaging or spectrosopic information about the object studied.
To ensure maximum power transfer between the RF power amplifier and the RF coil, as well as between the coil and the NMR receiver preamplifier, the coil input impedance must match the amplifier output impedance and the preamplifier input impedance. Additionally, the RF coil impedance must match the characteristic impedance of the transmission line coupling the RF coil to either the RF power amplifier or the preamplifier. This can be achieved by using adjustable impedance matching networks. A problem which arises with conventional NMR RF coil matching networks is that such circuits are dependent for proper operation on the quality factor, Q, of the coil which is subject to change with the size of the object in it. Thus, a large person (e.g., one weighing over 220 pounds) will lower the Q of the coil much more than a smaller person. A lower value of Q generally results in an impedance mismatch because a drop in the value of Q results in a concomitant reduction in the input impedance of the coil.
To regain optimum input impedance, conventional matching networks require either electrical connections to be moved, capacitor values substituted, or variable capacitors or inductors to be adjusted. However, the substitution of components having discrete values or the movement of electrical connections may permit only a finite number of NMR RF coil loads to be impedance matched. Equally undesirable is the use of adjustable capacitive and inductive elements near the bore of an NMR magnet due to their affect on the homogeneity of magnetic fields.
It is, therefore, a principal object of the present invention to provide an NMR RF coil impedance matching device which overcomes the shortcomings of the conventional device as discussed hereinabove.
It is another object of the invention to provide an NMR RF coil impedance matching device utilizing variable mutual inductance to effectively match a plurality of loads over a wide range of load impedances.